The air we inhale or live in contains 21% oxygen. If this percentage is increased it could lead to the formation of oxygen free radicals which are a highly reactive form of oxygen. Inside a living system, these free radicals can lead to membrane disintegration, protein breakdown, the slower exchange rate of gases, chest pain, and many more lethal issues. In bacteria nitrogen fixation occurs anaerobically and elemental oxygen can be poisonous for them.
